Phenol and methyl ethyl ketone are important products in the chemical industry. For example, phenol is useful in the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, alkyl phenols, and plasticizers, whereas methyl ethyl ketone can be used as a lacquer, a solvent and for dewaxing of lubricating oils.
The most common route for the production of methyl ethyl ketone is by dehydrogenation of sec-butyl alcohol (SBA), with the alcohol being produced by the acid-catalyzed hydration of butenes. For example, commercial scale SBA manufacture by reaction of butylene with sulfuric acid has been accomplished for many years via liquid/liquid extraction and gas/liquid absorption.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the cost of propylene relative to that for butenes is likely to increase, due to a developing shortage of propylene. Thus, a process that uses butenes instead of propylene as feed and co-produces methyl ethyl ketone rather than acetone may be an attractive alternative route to the production of phenol.
It is known that phenol and methyl ethyl ketone can be co-produced by a variation of the Hock process in which sec-butylbenzene is oxidized to obtain sec-butylbenzene hydroperoxide and the peroxide decomposed to the desired phenol and methyl ethyl ketone. An overview of such a process is described in pages 113-121 and 261-263 of Process Economics Report No. 22B entitled “Phenol”, published by the Stanford Research Institute in December 1977.
In addition, it is known that sec-butylbenzene can be produced by alkylating benzene with n-butenes over an acid catalyst. For example, in our International Patent Publication No. WO06/015826, we have described a process for producing phenol and methyl ethyl ketone, in which benzene is initially contacted with a C4 alkylating agent under alkylation conditions with a catalyst comprising zeolite beta or a molecular sieve of the MCM-22 family to produce an alkylation effluent comprising sec-butylbenzene. The sec-butylbenzene is then oxidized to produce a hydroperoxide and the hydroperoxide is decomposed to produce phenol and methyl ethyl ketone.
Although the chemistry involved in the alkylation of benzene with butenes is very similar to that for ethylbenzene and cumene production, as the carbon number of the alkylating agent increases, the number of product isomers also increases. For example, ethylbenzene has one isomer, propylbenzene has two isomers (cumene and n-propylbenzene), and butylbenzene has four isomers (n-, iso-, sec-, and tert-butylbenzene). For sec-butylbenzene production, it is important to minimize n-, iso-, tert-butylbenzene, and phenylbutenes by-product formation since these by-products, have boiling points very close to sec-butylbenzene and hence are difficult to separate from sec-butylbenzene by distillation (see table below).
ButylbenzeneBoiling Point, ° C.t-Butylbenzene169i-Butylbenzene171s-Butylbenzene173n-Butylbenzene183
Moreover, iso-butylbenzene and especially tert-butylbenzene are known to be inhibitors to the oxidation of sec-butylbenzene to the corresponding hydroperoxide, a necessary next step for the production of methyl ethyl ketone and phenol. However, although by-product formation can be minimized by using a pure n-butene feed, in practice it is desirable to employ more economical butene feeds, such as Raffinate-1, to produce sec-butylbenzene. A typical Raffinate-1 contains up to 2% butadiene and more than 5% isobutene. With this increased isobutene in the feed, a higher by-product make is expected even with a highly selective alkylation catalyst. Moreover, butadiene, if present, poses a significant problem since its alkylation product, phenylbutene, is an oxidation inhibitor and could react with another benzene to form diphenybutane byproduct.
There is therefore a need to provide a process for the production of sec-butylbenzene by alkylation of benzene using a mixed C4 olefin feed, such as Raffinate-1, in which the formation of undesirable by-products is significantly reduced by subjecting the feed to a pretreatment process that selectively reduces the level of isobutene and, where present, butadiene in the feed without excessive loss of valuable n-butenes.
International Patent Publication No. WO06/015826 discloses that isobutene in a mixed C4 olefin feed can be removed by dimerization or reaction with methanol to produce MTBE, prior to use of the feed in the alkylation of benzene to produce sec-butylbenzene. In practice, however, with Raffinate-1 and similar unprocessed C4 olefin feeds, we have found that using water and/or methanol as the reactive species, it is difficult in a single stage of contacting to secure sufficient conversion of isobutene to reduce the level of this isomer sufficiently to prevent significant iso-butylbenzene and tert-butylbenzene production in the subsequent alkylation step.